An organic semiconductor thin film can be formed by application of a solution. Therefore, an organic thin film transistor (organic TFT) is less expensive and has lower environmental loads than a conventional TFT using an inorganic semiconductor, and an electronic element having a large area can be produced using the organic thin film transistor. Furthermore, an organic device using the organic TFT is light in weight and flexible, and thus can be used widely in various devices such as a display and an IC tag.
However, the operation frequency of the current organic TFT is not yet sufficiently high compared with that of an inorganic TFT, and an enhancement of the operation frequency is desired. It is necessary to realize a short channel length L and a high mobility in order to enhance the operation frequency of a transistor. However, it is known that in fact, as the channel length L is shortened, the influence of contact resistance at the interface between an electrode and the organic semiconductor increases, and an effective mobility decreases.
For example, Patent Document 1 discloses that an alloy containing gold as a main component is used in a contact layer provided between source and drain electrodes that are made of gold and an insulating substrate in order to reduce contact resistance at the interface between the electrode and the organic semiconductor. That is, an alloy containing gold in an amount within a range of 67 atomic % or more to 97 atomic % or less is used.
Although gold often is used as a material for forming source and drain electrodes of an organic transistor of p-type operation, gold has low adhesion to other materials Therefore, a countermeasure is carried out in general in which a contact layer is formed between the gold electrode and the substrate. However, it has been pointed out that the contact layer causes parasitic resistance between the source and drain electrodes and the organic semiconductor layer.
To address this, Patent Document 1 states that it is possible to reduce parasitic resistance between the organic semiconductor layer and the electrodes by using the above-described alloy containing gold to form a contact layer.